The present invention relates in general to a device for the accurate and rapid measurement of complex reflection coefficients by the measurement of power levels at four detectors. The device described herein is constructed in a compact arrangement in either stripline, microstrip or waveguide. The device of this invention provides for the accurate and rapid measurement of complex reflection coefficients and, more particularly, the accurate measurements of low voltage standing wave ratios over waveguide bandwidths, or with regard to a stripline, or microstrip construction, over octave bandwidths.
With regard to the measurement of VSWR probably the most common approach is to use a directional coupler. However, in order to provide an accurate measurement, in particular of low VSWR, there is the requirement of a precise mechanical construction of the directional coupler. Therefore, such devices have to be made within quite tight tolerances.
Accordingly, one object of the present invention is to provide a device that makes either complex reflection coefficient measurements or low VSWR measurements without the need of a coupler or the like that requires tight construction tolerances.
One of the oldest techniques for the measurement of complex reflection coefficients (both amplitude and phase) is the slotted line apparatus. In this apparatus, the measurement is accomplished by means of a mechanical procedure which is quite time consuming and not at all suitable for computerization of the measured data.
Accordingly, another object of the present invention is to provide a device which will allow for the rapid computerized evaluation of data. In accordance with the invention this is accomplished by means of direct measurement of three normalized power levels. These power levels are then operated upon in accordance with the theory of this invention to provide readings of complex reflection coefficient or VSWR.
In another technique, the reflection coefficient is obtainable from the power measured at three fixed probes spaced one-eighth wavelength apart along a slotted line. See, for example, the article by W. J. Duffin, "Three Probe Method of Impedance Measurement", Wireless Engineer, Vol. 29, pp. 317-320 (Dec. 1952). However, with this technique, the reflection coefficient is obtained with a high degree of accuracy only over a narrow frequency range for which the one-eighth wavelength condition is substantially satisfied.
Accordingly, a further object of this invention is to provide a device for measuring the complex reflection coefficient accurately by the simple measurement of power levels over a full waveguide bandwidth, or in the case of stripline or coax over an octave or greater bandwidth.
One prior article by Henry J. Riblet "A Swept-Frequency 3-Centimeter Impedance Indicator", Proceedings of the I.R.E., Vol. 36, pp. 1493-1499, Dec. 1948 describes a device for determining the complex reflection coefficient in waveguide from the powers coupled to two detectors. However, with this technique, the measurement accuracy is very sensitive to the actual phase of the reflected signal.
Thus, another object of the present invention is to provide a device for the measurement of reflection coefficients wherein the measurement accuracy is essentially independent of both amplitude and phase.
An article by Henry L. Bachman, "A Waveguide Impedance Meter for the Automatic Display of Complex Reflection Coefficients", IEEE Trans. on MTT, Vol. 3, pp. 27-30, Jan. 1955 describes a waveguide impedance meter for the automatic display of complex reflection coefficients. However, the circuit configuration requires many waveguide components. Furthermore, it assumes for its operation a frequency independent 90.degree. phase shifter for which no practical realization exists. Also, the actual bandwidth is limited to 10%.
In another article by Glenn F. Engen, "An Improved Circuit for Implementing the Six-Port Technique of Microwave Measurements", IEEE Trans. on MTT, Vol. MTT-25, pp. 1080-1082, Dec. 1977 there is described a number of circuits for the accurate determination of the reflection coefficient over wide bandwidths from measurements of power levels at four detectors. Again, however, these circuits require many components.